Steel sheets treated by zinc-coating or zinc-alloy-coating are used to the corrosion-resistant parts in construction materials and materials for household electric appliances. That type of steel sheet coated by zinc-containing coating is generally used after painting, not in as-coated state. For painting, a pretreatment is generally applied. A common pretreatment is phosphate treatment.
The phosphate treatment is conducted by bringing a steel sheet coated by zinc-containing coating into contact with an acidic solution containing phosphoric acid ion, thus forming a crystalline film containing zinc phosphate as the main component onto the surface of the coated surface. The phosphate treatment improves the adhesion with the painting film, thereby attaining substrate performance stable to various paintings. Owing to the advantageous performance, the steel sheet treated by phosphate and coated by zinc-containing coating has been widely used as a substrate steel sheet accompanied with painting for construction materials, materials for household electric appliances, and the like.
Sole phosphate treatment, however, gives insufficient corrosion resistance because of the presence of residual micropores. Accordingly, generally the sealing is applied after the phosphate treatment to maintain the corrosion resistance. A conventional method of the sealing is to bring the steel sheet into contact with an aqueous solution containing hexavalent chromium by spraying, dipping, or the like, followed by drying the attached aqueous solution. Since, however, the hexavalent chromium is classified to an environmentally regulated substance, there is wanted a sealing without using the aqueous solution containing hexavalent chromium, or other method to improve the corrosion resistance.
Responding to the requirement, JP-A-2000-313967, (the term “JP-A” referred to herein signifies the “Unexamined Japanese Patent Publication”), for example, proposed a phosphate-treated zinc-coated steel sheet which is prepared by forming a conversion-treated film composed of a crystalline substance containing phosphoric acid onto the surface of a zinc-containing coating, and then forming an amorphous phosphoric acid film onto the conversion-treated film. As another example, JP-A-2004-143475 proposed a phosphate-treated zinc-coated steel sheet which has a sealing film prepared by forming a zinc-phosphate treated film onto the surface of a zinc-containing coated steel sheet, and then by applying an aqueous solution containing at least one metallic compound selected from the group consisting of a copper compound, a titanium compound, and a zirconium compound, or further containing a polycondensation resin compound of bisphenol-A, amine, and formaldehyde, onto the zinc-phosphate treated film, followed by drying the film. These disclosed technologies adopt sealing without using chromium.
All of the above-disclosed technologies, however, require heating to bake the applied aqueous solution during the step of forming the uppermost layer film. Consequently, these technologies have a drawback of necessity of an applying apparatus and a baking apparatus adding to the existed facilities to manufacture the phosphate-treated zinc-coated steel sheet, which increases the manufacturing cost.
There are trials for the technology to improve the corrosion resistance of the phosphate-treated film itself without sealing.
For example, JP-A-1-312081 proposed a metal material treated by phosphate and coated by zinc-containing coating, which is prepared by forming a zinc-containing coating layer onto the surface of a metallic material, and then by forming a film composed of a phosphate compound containing 0.1% by weight or more of Mg and preferably 5% by weight or less of Mg onto the coating layer. Furthermore, JP-A-2002-285346 proposed a steel sheet treated by zinc-phosphate and coated by zinc-containing coating, in which the zinc phosphate film on the zinc-containing coating layer contains 2% or more of Mg and 0.01 to 1% of at least one element selected from the group consisting of Ni, Co, and Cu, and the coating weight of the zinc phosphate film is 0.7 g/m2 or more.
According to these disclosed technologies, however, the phosphate film layer as the uppermost layer contains Mg so that there is a problem of discoloration of the surface to black, (hereinafter also referred to as “blackening”), when the steel sheet is exposed to a high temperature and high humidity environment. Furthermore, according to the last example technology, since the zinc phosphate film contains large amounts of Ni, Co, and Cu, there arises a problem of darkening the tone of the zinc phosphate film.
Responding to the problems of conventional technologies, the present invention has an object to provide a phosphate-treated zinc-coated steel sheet which does not apply sealing, which has corrosion resistance equivalent to that of the conventional phosphate-treated zinc-coated steel sheet processed by sealing, and which has excellent blackening resistance.